Method and penetrant for flaw detection



3,436,959 METHOD AND PENETRANT FOR FLAW DETECTION Clio lErnst Redemann, Monterey Park, and Robert A. Seybert, Huntington Beach, Calif., assignors to Purex fiorporation, Ltd., Lakewood, Calif., a corporation of California No Drawing. Filed Nov. 29, 1965, Ser. No. 510,362 Int. Cl. G01n 19/08 US. Cl. 73-104 8 Claims ABSTRACT OF THE DISCLOSURE 'Penetrant composition for application and entry into flaws in solid surfaces comprising monoalkyl benzene in which the alkyl group contains from 10 to 16 carbon atoms and a dye contained in the alkyl benzene to render detectable monoalkyl benzene located in said flaws.

This invention relates generally to improvements in the art of detecting flaws in solid surfaces, and is directed particularly to discoveries that have led to new penetrant compositions with superior functional properties and advantages for detection of flaws resulting from such conditions as excessive stressing of parts, corrosion, metal fatigue and the like.

Many types of penetrants have heretofore been used to locate flaws or discontinuities in metallic and nonmetallic surfaces. The usual method of application is to apply the penetrant to the surface and after a suitable time the excess penetrant is removed by washing with a suitable solvent system or mechanically by wiping with a cloth or other absorbent material; Any flaw is subsequently revealed by a seepage of the penetrant from the crevice or cavity. Various agents have been added to the penetrant to increase the visibility of the seepage. These include ordinarily visible dyes, fluorescent dyes. Absorbent powders are often applied to the surface of the article to absorb some of the dye and thus to increase the visibility of the flaw.

Various combinations of volatile and nonvolatile solvents have been used to provide the vehicle for carrying the dye or other indicator into the flaw. The requirements for a suitable solvent system have changed with the means of applying the penetrant, with the kind of indicator, and with the type of solid to which the process is to be applied. No single solvent has been found in the past which possesses all characteristics in an ideal balance and, therefore, systems of mixed solvents have been used in an attempt to compensate for the deficiencies in any of the known pure solvents. Systems based upon two or more solvents, one of which is relatively volatile and one which is relatively nonvolatile have been unsatisfactory when used in open tanks for dipping the articles into the penetrant. Because of one component being relatively volatile the composition of the mixture continuously changes during use and the performance of the system likewise changes.

It is the object of the present invention to provide for flaws detection by the penetrant-developer technique in which a penetrant is used which will enter the finest cracks, will drain properly from the exterior surface of the object, has an unusually high flash point, does not evaporate readily from an open tank, and remains or practically constant composition until fully used.

One of the primary requisites is that the solvent penetrates rapidly and completely into the narrowest crack or flaw. Prior inventors have operated under the premise that a solvent of high surface tension was required to fill nited States Patent 0 these cracks (similar to a high rise in a small capillary tube). However, since these cracks are always very narrow it appears that the ability of the liquid to wet the surface is even more fundamental. Harkins (The Physical Chemistry of Surface Films, New York, Reinhold Publishing Corp., 1952) has developed the following expression for the spreading coefficient of a liquid upon a solid surface.

S is the spreading coefficient of the liquid upon the solid 'y =free surface energy of the solid =interfacial surface tension of the liquid on the solid =interfacial surface tension of the liquid and its saturated vapor.

For spontaneous spreading of a liquid on a solid, S must have a positive value and the larger its numerical value the better the spreading will be. This would suggest that various solvent systems could be evaluated by the simple expedient of looking up values for the various constants in available tables. Actually this is not the case for one of these three quantities is practically indeterminable, namely 7 the free surface energy of the solid. This is because the exposed surface of a metal may be coated with oxides, may have an imperfect crystal structure, may be coated with impurities originally present in the metal which were pushed to the surface as the metal crystallized, etc. Quoting from Lee, Muir and Lyman (Journal of Physical Chemistry, vol. 69, p. 3220, 1965), Very little is known of the exact magnitude of the surface free energy of solids, even for simple ionic crystals. the general surface morphology, particularly the crystal structure, often predominates over the chemical constitution in determining the free surface energy in most solids. The best approach is in the direction of liquids which low surface tension and low interfacial surface tension with the surface to he wet. The final evaluation can be made however only by actual trial on the various classes of surfaces to which the penetrant is to be applied.

Once a liquid having suitable surface tension characteristics has been found, there are many additional conditions which must be satisfied before a marketable product can be produced. Some of the more important conditions are: The solution should have very little toxicity; it must not be too viscous or too fluid; it must dissolve sufficient dyestulf to perform well; it should have a high flash point and additionally be of low volatility; it should be stable to oxidation and photochemical decomposition; it should be non-corrosive with respect to the various surfaces to which it may be applied.

The present invention is predicated upon the discovery that a group of homologous organic liquids uniquely satisfy the criteria established for satisfactory performance of dye penetrant compositions. These are the monoalkylbenzenes in which the alkyl group has between 10 and 16 carbon atoms in the alkyl group and may be represented by where in has a value between 10 and 16 inclusive. The arrangement of the carbon atoms in the side chain may be variable, but the preferred configuration is where the carbon atoms are arranged in a linear configuration with the benzene nucleus attached at or near the end of the carbon chain.

The best performance of this system might be achieved with a completely pure isomer of a single composition. However, the present technology of making such alkylbenzenes does not permit the production of a completely pure single molecular species at an economically usable price. Consequently we are practically limited to commercially pure materials which in reality are narrow boiling range cuts containing one or more homologs and several isomers. These commercially pure materials are of such narrow boiling range they they behave almost as a single molecular species. Table I shows the physical properties of products offered by several manufacturers.

TABLE I Product A B O D E 245 247 262 265 12 l 13 13 0. 873 0. 874 0. 802 0.869 1. 487 1. 4877 1. 483 1. 486 Viscosity, centipoises, 20/C 5. 56 4. 51 7. 45 Bromine number, cgJg 0.05 0.02 0.05 0.02 0.02 Distillation range (A STMA, /C.):

% recovered 292 277 280 298 285 50% recovered 297 284 290 307 296 95% recovered 309 295 301 314 313 The boiling ranges do not appear to agree with the number of carbon atoms. Actually the difference in boiling ranges is presumably due to different isomer distribution, depending upon the particular manufacturing process used.

While the preferred embodiment of this invention employs the single solvent in commercially pure form, it has been found that these alkylbenzenes may be used in admixture with other solvents and for special purposes. These admixtures have advantages over mixtures currently known to the art. In particular admixtures of the alkylbenzenes with other non-oxygenated organic solvents such as light hydrocarbon oils including the aliphatics, both saturated and unsaturated, the cycloaliphatics, and the aromatics or various mixtures of these solvents, may be advantageous for particular surfaces or means of application to the surface. So varied, to suit particular purposes and conditions, may be the selection of such other solvents for admixture or solution with the alkyl benzene, that no attempt need be made to set forth, other than has been indicated, all possible solvent compositions, so long as it is understood that the alkyl benzene either essentially alone or in mixture or solution with other solvents, will be present in such amounts as to exert or contribute its uniquely effective surface tension characteristics and other properties mentioned in the foregoing. In general, it is contemplated that the alkyl benzene content of the penetrant product will range upward from about 50 %to around 98%.

The color component may be any suitable and preferably soluble dye in either the ordinarily visible or fluorescent categories, capable in minor percentage content of displaying distinct color contrast with the solid surface to which it is applied or with a developer applied to said surface. Such dyes are well known in the flaw detection art.

While capable of use for the detection of flaws in solid surfaces generally, the invetion has its principal usage for fiow detection in any of various metals and alloys. Accordingly it may most usually be desirable for the alkyl benzene to contain a minor percentage of either or both oxidation and corrosion inhibitors, that may be selected, as known in the prior art, according to the metallic surface being treated and related conditions. Such antioxidants soluble in the alkyl benzene may include any of the commonly employed hydrocarbon antioxidants generally used by the petroleum industry, examples of which may be pyrocatechol, N-butylated-p-aminophenol, 2,6-ditert.butyl-p-cresol, N,N-di-sec.butyl-p-phenylenediamine, butylated hydroxyanisole, etc.

Various corrosion inhibitors may be used depending, among other things, upon the kinds of metals to which the penetrant is to be applied. Typical materials are monoethanolamine, monoisopropanolamine, Z-amino-l-butanol, 2-amino-2-methyl-l-propanol, iso-butylamine benzoate, sodium naphthenates, mercaptobenzothiazole, dodecylamine, hexadecylamine, diethylamine ethoxyethanol, 1(2- hydroxyethyl)-2-heptadecenyl-2-imidazoline, octylphosphonic acid and its amine salts.

In general, the physical steps and operations employed in a practice of the invention may follow known techniques, see e.g. the Sockman and Brady U.S. Patent No. 2,667,070. Generally, the penetrant is applied, heated or unheated, to the heated or unheated metal surface to cause effective penetration of any flaws present. Thereafter the penetrant is removed from the general surface area so that penetrant to exude from the flaws becomes visible in color contrast with the surface metal. Commonly, cleaning of the surface is followed by application of so-called developer, e.g. powdered or other absorbent coating to the surface area to be investigated. Withdrawal of dyed penetrant from the flaws into the absorbent enables the latter to display the location, shape and extent of the flaws.

The following examples may be taken as typical of the invention but do not necessarily limit the invention to these compositions. Various dyes, both visible and the so-called fluorescent ones, may be employed to indicate visually the location of the flaw or crack.

Example I Percent Dodecylbenzene 97.75 Red dye, Color Index #26125 2.00 Monoethanolamine 0.25

Total 100.00

Example II Tetradecylbenzene 97.80 Red dye, Color Index #26110 2.00

Total 100.00

Example III Decylbenzene 98.50

Methyl Violet Base A, Color Index Solvent Violet Such formulations may be proposed and used in any suitable form and manner. For some purposes it may be desirable to package the penetrant for aerosol application, in which instances the penetrant will be put into conventional dispensing valved aerosol containers charged with suitable propellant such as selected Freons, or normally gaseous hydrocarbons where use conditions permit. As before the penetrant will be composed of the alkyl benzenes with dissolved dye and either or both oxidation and corrosion inhibitor.

As herein used, the term flaws is intended to be inclusive of any of such surface defects as cracks, crevices, cavities, pits and the like, which permit entry of an indicator penetrant.

We claim:

1. Penetrant composition for application and entry into flaws in solid surfaces, consisting essentially of monoalkyl benzene in which the alkyl group contains from 10 to 16 carbon atoms, and an indicating amount of a dye contained in the alkyl benzene.

2. Penetrant composition according to claim 1, containing also a corrosion inhibting amount of a metal corrosion inhibitor.

3. Penetrant composition according to claim 2, in which the dye and corrosion inhibitor are dissolved in the alkyl benzene.

4. Penetrant composition according to claim 1, in which the alkyl benzene is essentially dodecyl benzene.

5. Penetrant composition according to claim 3, in which the alkyl benzene is essentially dodecyl benzene.

6. The method of detecting crevice-like flaws in a solid surface that comprises applying to said surface and into the flaws monoalkyl benzene in which the alkyl group References Cited UNITED STATES PATENTS 2,340,940 2/ 1944 De Forest 73-104 2,420,646 5/1947 Bloom et al. 73-104 2,667,070 1/1954 Stockman et al. 73-104 3,028,338 4/1962 Parker 252301.2

JAMES J. GILL, Primary Examiner.

H. GOLDSTEIN, Assistant Examiner.

U.S. Cl. X.R. 

